Such a series current regulator is, for example, of interest in controlling the rate of charging of the battery of power cells from a solar array in a space vehicle, where the end-of-charge battery voltage closely approaches the voltage supplied from the solar cell array. Such a series voltage regulator is, for example, of interest following a switching voltage regulator to suppress remnant switching ripple and audio-frequency noise.
The efficiency of a regulator is determined by the ratio of the power it delivers to the power it receives, percent efficiency being one-hundred times that ratio. Series regulation, as opposed to shunt regulation, tends to be more efficient. This is especially so as the difference between regulator input and output voltages becomes increasing less than output voltage.
In a series regulator wherein the difference between input and output voltages is substantially less than output voltage, a substantial portion of its inefficiency can be attributable to power consumed in implementing the control of conduction through the series-pass element that is used to provide the controlled voltage drop between regulator input and output voltages. A bipolar transistor having its collector/emitter path used as a series-pass element requires base current for control, tending towards considerable power consumption in the base current drive circuitry. A series-pass element provided by Darlington connection of bipolar silicon power transistors will require proportionally less control current, but cannot regulate regulator output voltage closer than a volt or so to regulator input voltage. A punch-through or super-beta bipolar silicon power transistor used as series-pass element also has proportionally less control current, but is susceptible to punchthrough of regulator input voltage transient spikes and possible damage therefrom.
Using the channel of a field effect transistor as the series-pass element in a series regulator is advantageous, from the standpoint of regulator efficiency, in that the gate electrode consumes no steady-state power. This is so even where the current flow through the series-pass element is in the several ampere range, reaching up to levels where the channels of a plurality of power FET's have to be parallelled to provide the series pass element. Power FET's of such current handling capability presently available are vertical-structure devices and are only of n-channel enhancement-mode type. Gate cut-off voltage is one volt or more, and typically is several volts.
Where output voltage approaches the input voltage in a series regulator using the channel of such an enhancement-mode power FET in its series-pass element, the gate voltage required to place the channel well into conduction will substantially exceed the input voltage to, and output voltage from, the regulator.